Formation fluid sampling by Wireline Formation Tester (WFT) during drilling operation represents an important component of the formation evaluation system established by the petroleum industry, especially when it deals with high profile and offshore wells. It is well known that the errors in estimates of formation fluid properties can lead to significant miscalculations in design and performance prediction of flow assurance, well construction, and production facilities. The main challenge in obtaining representative samples of formation fluid by WFT is related to the mud filtrate invasion during drilling. After a few hours of drilling, the borehole is usually surrounded by the invasion zone saturated predominantly with mud filtrate and, for this reason, any sampling operation launched during interruption of drilling has to start from the cleanup production, which continues until the target of contamination tolerance is reached or the time allocated for the sampling operation has run out. The major challenge of cleanup production monitoring represents the case of drilling with oil-based mud (OBM) due to miscibility of OBM filtrate with formation hydrocarbons and poor resistivity contrast.
The variation of formation fluid properties during cleanup production, however, can be reliably detected by optics. Existing Optical Fluid Analyzers (OFA) can measure the optical density (OD) of produced mixtures in a wide spectrum of invisible light with the wavelengths in the range from 400 nm to 2200 nm. In presence of initial OD contrast between the OBM filtrate and the formation fluid, the high sensitivity of optical measurements to the composition of produced fluid can be observed. This sensitivity, however, does not allow for the quantification of contamination in the produced fluid, since the OD of virgin formation fluid is unknown in advance. To compensate for the lack of information during cleanup production monitoring, the contamination transport prediction has to be involved to achieve the closure of optical monitoring model.
Although advanced sampling operations with optical contamination monitoring have been used by the industry for more than ten years, the contamination transport modeling for cleanup prediction is still in its rudimentary phase. The approach to the contamination transport during cleanup production, which is currently considered as state of the art by the industry, is based on the empirical model for the contamination evolution with time during the late phase of cleanup. This model states that the contamination, defined as the concentration of mud filtrate in the produced fluid, has to decrease with time as t−5/12. This behavior contradicts the analytical model, which is based on the pseudo-spherical flow and contamination transport, predicting faster decay of contamination η with time, namely as t−2/3. Recently, the numerical simulations have confirmed the empirical law, η˜t−5/12, but have not explained the existing contradiction between the theory and field observations.